The　effects　of　ultrasonic　treatment　on　the　microstructure　and　mechanical　properties　of　Mg-5Zn-2Er　alloy　at　room　temperature　(RT)　and　high　temperature　(HT)　were　investigated.　The　microstructure　and　mechanical　properties　of　the　samples　were　studied　by　OM,　SEM　and　MTS　material　tester.　The　results　show　that　the　microstructure　and　mechanical　properties　are　improved　after　the　ultrasonic　vibration.　The　best　effects　of　ultrasonic　vibration　on　microstructure　and　mechanical　properties　were　obtained　with　the　ultrasonic　vibration　power　of　600　W　and　time　of　100　s.　The　cavitation　and　acoustic　streaming　caused　by　ultrasonic　treatment　play　a　major　role　in　refining　the　microstructure　and　increasing　mechanical　properties　of　the　alloy.